Typically, a crankcase and a cylinder block forming a multiple-cylinder four-cycle engine equipped in a vehicle such as a motorcycle are manufactured by casting. In case of the motorcycle, the engine is mounted to a frame member disposed between a front wheel and a rear wheel. The engine and a transmission are mounted in a limited space below a fuel tank mounted to an upper region of the frame member to drive the rear wheel. So, a cylinder of the engine is positioned on the front wheel side and the transmission is positioned on the rear wheel side. The engine is typically manufactured in such a way that the cylinder block, a transmission case and a crankcase are integrally cast by die casting.
In the multiple-cylinder four-cycle engine, since crank chambers of respective cylinders are separated by separating walls, air in each crank chamber is compressed by a piston moving downward, causing a pumping loss. In addition, a mixture of air and oil, namely, an oil mist, existing in the respective crank chambers, is scattered by a crank web or the like rotating at a high speed in the crank chambers to cause a large friction resistance. In some engines, crank chambers of adjacent cylinders are connected to each other through a connecting hole in order to reduce the pumping loss or the friction resistance.
Prior art that was configured to address these concerns is disclosed by Japanese Laid-Open Patent Application Publication No. Hei. 11-182325. According to this application, connecting holes connecting crank chambers of cylinders are formed to extend over piston sliding surfaces inside cylinders and crankcase parts, and a center of each connecting hole is located on the sliding surface side of the piston.
A similar prior art concept is disclosed in Japanese Laid-Open Patent Application Publication No. 2005-69170 which was previously filed by the applicant. In this application, connecting holes are formed to penetrate separating walls defining cylinder bores and crank chambers of adjacent cylinders.
A multiple-cylinder four-cycle engine for motorcycles in some cases has a cylinder block having a closed deck structure in which an upper region of a water jacket formed in a cylinder block is closed, in order to increase a degree of roundness of an inner diameter of the cylinder at an upper region of the cylinder block. However, since it may be difficult to manufacture via die casting, a cylinder block having a closed deck structure is generally manufactured via sand casting using a sand mold.
FIG. 6 is a perspective view showing a prior art example of an upper crankcase 52 of a crankcase 51 of an engine for a motorcycle. As shown in FIG. 6, the upper crankcase 52 is inverted in casting. The upper crankcase 52 has a cylinder block 53 having the above mentioned closed deck structure and an upper transmission case 54 which are integrally cast and formed by using a sand mold.
FIG. 7 is a longitudinal sectional view schematically showing the upper crankcase 52 of FIG. 6 and a casting mold 60 for casting the upper crankcase 52. As shown in FIG. 7, a casting mold 60, which is a sand mold, includes a lower die 61, a front die 62, a side die (located in a direction perpendicular to a sheet of FIG. 7 and thus is not shown) provided at a side surface, a rear die 63, a pouring gate die 64 forming a pouring gate 65 from which liquid metal is fed into the casting mold 60, an upper die 70, and a cylinder bore die 66. The casting mold 60 further includes a crankshaft side core 69 and a cylinder side core 68, to form intricate inner wall surfaces of crank chambers 56 (FIG. 6) of the upper crankcase 52.
FIG. 8 is a cross-sectional view taken along line VIII-VIII of FIG. 7, schematically showing the casting cores 68 and 69 for forming the crank chamber 56 (FIG. 6) of the crankcase. As shown in FIG. 8, the crankshaft side core 69 is provided in contact with the upper die 70 at a joint surface 55, and a cylinder side core 68 is provided on the cylinder side with respect to the crankshaft side core 69 in contact with the crankshaft side core 69 at a parting plane 75. The cores 68 and 69 respectively have drafts 80 and 81 having a width that decreases from a joint surface 55 of the crankshaft side C toward the cylinder side H.
As shown in FIGS. 6 and 7, a connecting hole 58 is formed on each separating wall 57 for defining the crank chamber 56 of the upper crankcase 52 to reduce friction resistance and the like within each crank chamber 56. In addition, a cut portion 78 is formed on each separating wall 57 to reduce the weight of the upper crankcase 52. The cut portion 78 may be formed on each separating wall 57 between adjacent cylinders by a shape of the sand mold of the cylinder side core 68.
In casting, flash may be left at an end portion E of the parting plane 75. The flash must be removed to smooth-finish the inner wall surface, or the like, of the crank chamber 56. However, in the prior art discussed above the parting plane 75 is positioned in close proximity to a joint portion at which the upper crankcase 52 is joined to the cylinder block 53, (i.e. in a narrow space) and cut portion 78 is located closer to the cylinder side H than parting plane 75. For these reasons, an operator may have to take painstaking efforts to carefully remove the flash by hand. In order to smooth-finish corner regions of the cut portions 78 and surfaces of the separating walls 57, the operator must remove the flash not only in the vicinity of the parting plane 75 but in the vicinity of the cut portions 78 existing in the narrow spaces.
In addition, it will be appreciated that it may be desirable to increase the output of high-power engines such as the above described engine for motorcycles, and that it may also be desirable to increase the output of multiple-cylinder four-cycle engines for motorcycles.